Regarding Heathrow...I think the biggest issue is that the weather station is sandwiched on a small area of grass between the north runway (one of the busiest runways in the world) and the northern perimeter roads and car parks. So it's between two large slabs of black tarmac. In addition, being in the northern side of the airport means that in any hot southerly, it's just advecting all of the heat from the airport towards the weather station. Anyone who has been to Heathrow will know that it's basically the size of a town in itself.
I think if you moved it to a patch of grass in the southwest corner of the airport you'd see some decrease in the summer maxima.